1. Field of the Invention
This invention relates to silicon sensors such as pressure sensors and accelerometers which include resistors having a resistance value that varies with the applied force, and which are made by semiconductor processes.
2. Description of the Prior Art
Silicon pressure sensors and accelerometers generally use a diffused or implanted Wheatstone bridge to convert applied force to an electrical signal. Sometimes such sensors use a half bridge, with half the sensitivity. The resistors of the bridge form a network and are positioned on a thin silicon diaphragm in such a way that their values change in opposite directions with force applied to the diaphragm, thus creating a resistance that results in a usable output signal in response to an applied voltage. FIG. 1 shows an example of a prior art silicon sensor with four resistors R1-, R2+, R3-, R4+ with arrows 8-1, 8-2, 8-3, and 8-4 that show the direction of resistor value change with force applied from above. "Up" arrows 8-1 and 8-3 indicate that resistors R2+ and R4+ increase resistance with applied force; "down" arrows 8-2 and 8-4 indicate that resistors R1- and R3- decrease resistance with applied force. In FIG. 1, the four resistors R1-, R2+, R3- and R4+ are located on silicon diaphragm 10 which is part of silicon chip 12. FIG. 2 shows schematically how these same four resistors R1-, R2+, R3-, R4+ would be arranged in a prior art Wheatstone bridge to maximize the useful output voltage [(V.sub.out +)-(V.sub.out -)] resulting from an applied force. V+ represents a supply voltage to ground (GND). For the Wheatstone bridge, all four resistors R1-, R2+, R3-, and R4+ have the same resistance in the absence of an applied force. Note that for R1- and R3-, a force applied to silicon diaphragm 10 lessens the resistance; for R2+ and R4+, such a force increases resistance, as shown by the direction of the arrows through the resistor symbols.
An ideal sensor device of the type shown in FIG. 1 would have a zero differential output voltage [(V.sub.out +)-(V.sub.out -)] with no force applied, a specified differential output voltage response per unit applied force, and both features would not change over the operating temperature range. The prior art technology approximates the ideal device by adding resistor networks in series and in parallel with the basic four resistor bridge device. This network is then laser trimmed to achieve a satisfactory performance compromise. The added resistor networks can be thick or thin film resistors connected to the sensor, or thin film resistors can be fabricated on the sensor chip. The laser trim approach adds to processing costs for the film depositions and patterning, and laser trimmers are expensive and costly to maintain. In addition, the trimming must be done before final assembly of the package; thus stresses induced by final assembly can undo the work of the laser trimmer.
It would therefore be advantageous to be able to trim a sensor after assembly and to do so without the additional costs associated with laser trimming.
Other known methods to trim resistors include Zener diode "zapping" and fuse blowing. Both have many of the same disadvantages as does laser trimming.
Vyne, in U.S. Pat. No. 4,606,781, issued Aug. 19, 1986, discloses another method for trimming an electrical component that is located within an integrated circuit and, more particularly, to a method for trimming implanted or diffused resistors by the use of metal migration. A resistor is constructed as part of an integrated circuit by implanting or diffusing dopants into a semiconductor material through apertures in a masking material. The two electrical contacts to the resistor required to allow electrical current to flow through the resistor are formed by first depositing an insulating layer over the resistor region. Then two openings, called preohmics, are etched into the insulating layer, one at each end of the resistor. A layer of metal is deposited over the insulating layer and into the two preohmics, thus making contact with the two ends of the resistor. The layer of metal is selectively etched to leave two regions of metal, one region overlapping each preohmic. The shape of each preohmic defines the shape of the metal contact.
Pulsating a direct current through the metal contacts of the diffused resistor results in a metal filament controllably migrating from the positive contact to the negative contact. As the metal migrates through the silicon, the value of the resistor changes, decreasing as the metal filament approaches the negative contact. A contact with a sharp corner (such as a contact with a rectangular shape in the plane of the surface of the sensor) or a corner with a small radius of curvature will allow the metal to migrate with less current than, for example, a circular shaped contact with a large radius.
A resistor is trimmed (i.e., the resistance of the resistor is adjusted downwards) by this method to a given value by adjusting the duration and amount of current passing through it as well as by adjusting the number of current pulses. See Vyne, supra, at col. 4, lines 1-18. It is possible to completely short the resistor by pulsing the current for an extended length of time.